Newton's first law is, unfortunately, not a trivial proposition. What it tells us is that there are inertial systems in which the motion of objects is, as my high school physics teacher put it, "as simple as it can be". The existence of such systems is an empirical observation, even though it is a non-trivial one on the surface of Earth which is obviously not an inertial system.
It is important to train students to accept empirical information as the guiding principle in physics. They have to understand that physics is not mathematics. It does not "derive" its results from some abstract axioms. Everything we do in physics starts and ends with observations and experiments.
The primary, I would even say primal observations that "things at rest tend to stay at rest" and "things in motion follow a straight line" have to be demonstrated experimentally beyond any doubt. That is what the air-table experiments are for. They are not just a sixty-second demonstration of the first law. They are one form of experimental evidence for the first law and ideally students should be allowed to make this experience hands on.
In other words: the work of the physics teacher does not start by introducing laws and formulae that the students have to learn to pass the exams. It starts by actually demonstrating physics hands-on and by giving students the confidence to trust their observations. The question "Why is this so?" has to be replaced by "I have seen with my own eyes that it is so!". Only if we see an air puck that moves in a straight line over and over again can we learn to trust that "nature likes straight lines". Only when the confidence in these observations is gained can the student be taught how to analyze those straight line motions and how to make good use of them for more complicated scenarios.
There is, of course, a higher level of theoretical mechanics that explains exactly "Why this is so.", but I doubt that you will be successful in teaching the majority of high school students about the least action principle, Noether's theorem and differential geometry. Maybe a few of your students will go to university and learn the modern perspective on Newton's laws, but they can only do that successfully if you instill the necessary trust in them that "If nature says so, then it is so.".